1. Field of the Invention
The present invention relates to a thin film semiconductor device used in a display, an image scanner, or the like, and to a method of manufacturing the same and, more particularly, to a thin film semiconductor device having uniform electrical characteristics and high reliability upon an increase in chip size in a thin film transistor and a thin film transistor type photosensor.
2. Related Background Art
Along with recent office automation, input/output devices such as a display and an image scanner have received a great deal of attention as man-machine interfaces of OA equipment such as a word-processor, a personal computer, and a facsimile machine. Strong demand has arisen for lightweight, low-profile, low-cost input/output devices.
Judging from these viewpoints, a thin film semiconductor such as hydrogenated amorphous silicon or polysilicon is formed on an insulating substrate having a large area, and an active matrix liquid crystal display using thin film transistors or a photoelectric transducer apparatus using a photosensor have been extensively developed.
FIG. 1A is a sectional view illustrating a structure of a conventional thin film transistor (to be referred to as a TFT hereinafter).
A gate insulating film 2 is deposited on a gate electrode 1, and a thin film semiconductor layer 3 consisting of, e.g., hydrogenated amorphous silicon (to be referred to as a-Si:H) and serving as a channel region is formed on the gate insulating film 2.
N.sup.+ -type layers 6 are formed between the thin film semiconductor layer 3 and metal electrodes as source and drain electrodes 4 and 5 to serve as ohmic junctions for electrons and blocking junctions for holes. The resultant transistor operates as an n-channel transistor. This transistor structure has a thin film semiconductor surface defining the upper surface of the channel region. FIG. 1B is a plan view illustrating the transistor structure of FIG. 1A. FIG. 1B particularly illustrates a TFT having a planar (the electrodes 4 and 5 in FIG. 1B have an interdigital structure) electrode structure proposed to prevent an increase in channel length and solve the conventional fabrication problems.
The TFT shown in FIGS. 1A and 1B can be applied as a secondary photocurrent type photosensor (e.g., Japanese Laid-Open Patent Application No. 60-101940).
FIGS. 2A and 2B are views for explaining the steps in manufacturing the conventional TFT shown in FIGS. 1A and 1B (the method of manufacturing this TFT is shown in, e.g., Japanese Laid-Open Patent Application No. 63-9157).
A substrate comprises a glass substrate G, and Cr serving as a gate electrode 1 is formed on the glass substrate G. Cr is selectively etched by a photolithographic technique to form the gate electrode 1. A 3,000-.ANG. thick silicon nitride film 2 serving as a gate insulating film, a 5,000-.ANG. thick a-Si:H layer 3 serving as a semiconductor layer, and a 1,500-.ANG. thick n.sup.+ -type layer 6 are continuously formed on the glass substrate G by, e.g., a plasma CVD method. Aluminum serving as source and drain electrodes 4 and 5 is then deposited by sputtering. A photosensitive resin 8 is applied to the entire surface (FIG. 2A). The resin 8 is patterned into a desired shape upon exposure and development. The aluminum layer serving as the source and drain electrodes is patterned using the resist pattern (FIG. 2B). In this case, the photosensitive resin 8 as a resist pattern is present on the electrodes. The n.sup.+ -type layer is etched to a depth of 1,800 .ANG. by etching such as RIE (Reactive Ion Etching) using the photosensitive resin as a mask. The photosensitive resin is then removed. Interelement isolation between TFTs is performed to prepare each TFT in FIG. 1A.
After the above process, the surface of a semiconductor thin film of a conventional thin film transistor is susceptible to the influence of an outer atmosphere. When oxygen gas or steam is directly brought into contact with, and adsorbed and diffused in the surface of the semiconductor thin film, electrical characteristics of the semiconductor thin film vary because the semiconductor thin film has a very small thickness. For this reason, it is proposed to cover the element surface with a protective film consisting of silicon nitride (Si.sub.3 N.sub.4), aluminum oxide (Al.sub.2 O.sub.3), or silicon oxide (SiO.sub.2) (e.g., Japanese Laid-Open Patent Application No. 59-61964).
A method using a polyimide resin film polymerized in a heat treatment as a protective film is also proposed.
In order to further improve stability of the element, a method of stacking a second protective film of the same material as that constituting the thin film semiconductor layer 4 on the polymerized polyimide resin film is also proposed (e.g., Japanese Patent publication No. 1-137674).
A plurality of thin film transistors and photosensors formed on a large substrate by the method described with reference to FIGS. 2A and 2B are required to exhibit uniform characteristics within the large substrate. In the thin film transistor or photosensor formed in the process of FIGS. 2A and 2B, particularly when RIE (Reactive Ion Etching) is used in the step of etching the n.sup.+ -type layer in FIG. 2B, the surface of the thin film semiconductor layer is damaged by incident ions and the like by RIE, and the electrical characteristics are degraded. In addition, uniformity of the electrical characteristics tends to be lost by a distribution or the like of the RIE incident ions. For example, a threshold voltage which determines the operating characteristics of the thin film transistor varies within the range of several volts on the substrate, and a decisive drawback may be present in an actual application. For example, a display state is greatly changed in an active matrix display. In a sensor, the photocurrent and the dark current as the basic characteristic values greatly vary between elements, and the quality of the read image is greatly degraded, thus posing decisive problems in basic performance.
When a protective film on TFT photosensors having nonuniform characteristics consists of an organic material such as polyimide, stability such as sufficiently high humidity resistance against environmental conditions cannot be expected.
On the other hand, when the protective film consists of an inorganic material (e.g., a-SiN:H) which directly contacts the semiconductor, undesirable electrical characteristics or characteristic distributions are caused in accordance with the formation process of this protective film and the composition of the resultant protective film. For example, Hiranaka et. al. reported the relationship between the composition of the insulating layer and the thin film semiconductor layer 3 as a problem of a TFT gate interface. More specifically, according to Hiranaka et. al., as a problem of the gate interface between the gate insulating film 2 (SiN.sub.x :H) and the thin film semiconductor layer 3 (a-Si:H), the gate insulating film composition largely influences a band state of the thin film semiconductor layer 3 (J. Appl. Phys. 62(5), from P. 2129 (1987) and J. Appl. Phys. 60(12), from P. 4294 (1986)). The composition of the insulating layer as a protective film is also assumed to greatly influence humidity resistance.
A fabrication process of a conventional thin film transistor will be described with reference to FIGS. 3A to 3F. A gate electrode 302 is selectively formed on an insulating substrate 301. A gate insulating film 303, a thin film semiconductor layer 304, and an n.sup.+ -type layer 305 are sequentially deposited on the insulating substrate 301 by the plasm CVD method (FIG. 3A).
An electrode layer 310 serving as source and drain electrodes 306 and 307 is deposited, and a photoresist 308 for patterning the source and drain electrodes 306 and 307 is applied to the electrode layer 310 (FIG. 3B).
The photoresist is patterned into a desired pattern, and the electrode layer 310 is etched by wet etching using the photoresist 308 as a mask, thereby forming the source and drain electrodes 306 and 307 (FIG. 3C).
The n.sup.+ -type layer 305 is etched using the photoresist 308 as a mask, and the photoresist 308 is removed (FIG. 3D).
After the n.sup.+ -type layer 305 is patterned to have a desired shape by using the photoresist pattern as a mask, the insulating layer 303, the thin film semiconductor layer 304, the n.sup.+ -type layer 305, and the electrode layer 310 are etched to perform element isolation (FIG. 3E).
A protective layer 309 is formed on the surface of the thin film semiconductor element by the plasma CVD method to finish the element (FIG. 3F).
The conventional thin film transistor type photosensor is manufactured as described above.
In the thin film transistor and the thin film transistor type photosensor formed by the method described with reference to FIGS. 3A to 3F, when the n.sup.+ -type layer is etched by reactive ion etching (to be referred to as RIE hereinafter), the surface of the thin film semiconductor layer is damaged by ions incident on the surface of the semiconductor surface. The important electrical characteristics of the thin film transistor type sensor may therefore be greatly degraded. When such an element is formed on a large substrate having a large size, the electrical characteristics of the thin film transistor and the thin film transistor type photosensor have a distribution having a wide range.
In addition, when the surface protective layer of the thin film semiconductor is consists of a silicon nitride film, electrical characteristic degradation such as an increase in dark current in the n-type thin film semiconductor surface may occur. As result, the conventional thin film transistor and the conventional thin film transistor type photosensor cause degradation of display characteristics in a display and degradation of gradation by a low S/N ratio in a sensor in accordance with the degradation of the electrical characteristics and their wide distribution. In this manner, decisive problems associated with basic performance of the element often occur.